Crystal water as the mol­ecular glue for obtaining different co-crystal ratios: the case of gallic acid tris-caffeine hexa­hydrate

The crystal structure of the hexa­hydrate co-crystal of gallic acid and caffeine, C(7)H(6)O(5)·3C(8)H(10)N(4)O(2)·6H(2)O or GAL3CAF·6H(2)O, is a remarkable example of the importance of hydrate water acting as structural glue to facilitate the crystallization of two components of different stoichiometries and thus to compensate an imbalance of hydrogen-bond donors and acceptors. The water mol­ecules provide the additional hydrogen bonds required to form a crystalline solid. Whereas the majority of hydrogen bonds forming the inter­molecular network between gallic acid and caffeine are formed by crystal water, only one direct classical hydrogen bond between two mol­ecules is formed between the carb­oxy­lic oxygen of gallic acid and the carbonyl oxygen of caffeine with d(D⋯A) = 2.672 (2) Å. All other hydrogen bonds either involve crystal water or utilize protonated carbon atoms as donors.